1. Field of the Invention
The invention relates to a calibration method and system for use in an induction motor torque controller.
2. Background Art
Electric multi-phase machines have been developed for use in automotive vehicle drivelines including electric vehicles, hybrid vehicles having an internal combustion engine and an induction machine, and fuel cell electric vehicles. Such machines also have been designed for other uses such as for integrated starter/generator units and electric power-assisted steering gears for automotive vehicles. An advantage in using electric machines for such applications include the ability to accommodate increased vehicle electric loads, a potential increased fuel economy, and reduced engine exhaust emissions. Induction machines for these applications are attractive because of their rugged nature and low cost.
The increased use of induction machines for automotive purposes increases the need for self-calibration of the controller. This is desirable because of the inherent variations of the characteristics of the individual machines and the changes in the operating environment such as ambient temperature changes.
In the co-pending patent application identified above, a method and strategy is disclosed for obtaining an estimate of the slip gain and rotor time constant. The slip gain value is a key factor in the performance and efficiency of an indirect fuel-oriented controller, as described in the co-pending application. The advantage of the estimating technique is that it is capable of providing direct estimates of the slip gain independently of any other machine or system parameter. Further, it does not require direct measurement of the torque produced by the machine. The method described in the co-pending application makes possible a tuning of the slip gain using only the sensors and components that already are present in the electric machine drive, such as a speed sensor or a rotor position sensor.
The slip gain and rotor time constant estimation method disclosed in the co-pending application is based on the transient response of the stator voltage after a change in the torque command. The control method does not require a direct measurement of torque being produced by the machine.
The calibration or tuning of an indirect field-oriented machine drive embodying the present invention can be divided into distinct levels.
At a high level, the calibration procedure used for estimating the slip gain or rotor time constant needs to be repeated for both motoring and generating operation at each of the rotor flux levels and rotor speeds of interest. The slip gains obtained from this test sequence can then be used to form, or make corrections to, a lookup table, a curve-fit, or some other mapping for use during the normal operation of the induction machine drive.
The amount of testing performed at this level depends on the amount of variability seen between individual machines, the reliance on test results from previous machines and test procedures, and the degree of tuning accuracy desired. When a high level of detail and accuracy is required, the self-commissioning procedure would perform the calibration at numerous test points covering the whole operating range of drive.
When just a fine tuning of previously measured gains is desired, which may have been obtained from tests on a different machine, the calibration may be performed at only a limited number of test points. The results of the tuning from these test points would then be extrapolated to correct the gains at the other untested operating conditions. In either case, the calibration procedure for the slip gain estimate should be accurate, reliable and fast. The estimation procedure based on the transient response of the machine to changes in the torque command fits these requirements.
In practicing the invention, the induction machine drive would have a rotor and a fixed stator that forms a torque flow path from a torque input member to a torque output member. The method of the invention comprises the steps of commanding a change in torque on the torque input member, determining commanded q-axis and d-axis stator voltages, calculating the product of the stator voltage and a desired windowing function. The windowing function is used as a multiplier for stator voltage data following a change in a torque command to effect a correctly tuned slip gain estimate.
Parameters, called q and d factors, are calculated for a current transient event as a time integral of a windowed q and d axis stator voltage waveform. Using the q and d factors, a determination is made with respect to whether the estimate of slip gain is accurate. The slip gain estimate then is corrected for a given change in commanded torque, if necessary, based on the q and d factors relative to a slip gain estimate for a correctly tuned drive.
The torque command is repeatedly varied between predetermined values with a predetermined frequency, thereby creating a torque command waveform that causes the slip gain estimate to converge to an accurate, unchanging value.